Public Release: 31-Jul-2014
Symbiotic survival

IMAGE: This is a schematic cutaway view of the lucinid genus Codakia in a seagrass bed.
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Credit: S.M. Stanley, Fig 2. Geology.

Boulder, Colo., USA - One of the most diverse families in the ocean today -- marine bivalve mollusks known as Lucinidae (or lucinids) -- originated more than 400 million years ago in the Silurian period, with adaptations and life habits like those of its modern members. This Geology study by Steven Stanley of the University of Hawaii, published online on 25 July 2014, tracks the remarkable evolutionary expansion of the lucinids through significant symbiotic relationships.

At is origin, the Lucinidae family remained at very low diversity until the rise of mangroves and seagrasses near the end of the Cretaceous. According to Stanley, the mangroves and seagrasses created protective habitats in which the bivalve mollusks could thrive, in turn providing benefit through a sort of tri-level symbiosis.

Stanley writes that what was especially important was the lucinids' development of a symbiotic relationship with seagrasses. The lucinids flourished as they took advantage of the oxygen-poor, sulfide-rich sediments below roots and rhizomes. These habitats provided a rich supply of sulfur-oxidizing bacteria (or endosymbionts), which the bivalves "farmed" on their gills and then consumed. At the same time, the seagrasses benefited from the uptake of (to them) toxic sulfide by the bivalves.

The Cretaceous mass extinction, which killed off not only the dinosaurs but also many forms of marine life, had little impact on the lucinids. Stanley writes that this can be attributed to the fact that the bivalves relied heavily on the endosymbiont bacteria for nutrition at a time when productivity of marine algae collapsed and many suspension-feeding groups of animals died out. About 500 lucinid species exist today, with by far the highest diversity in shallow-sea seagrass meadows.

FEATURED ARTICLE
Evolutionary radiation of shallow-water Lucinidae (Bivalvia with endosymbionts) as a result of the rise of seagrasses and mangrovesSteven M. Stanley, University of Hawaii, Dept. of Geology and Geophysics, POST Building 701, 1680 East-West Road, Honolulu, Hawaii 96822, USA. Published online 25 July 2014; http://dx.doi.org/10.1130/G35942.1.

Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to Geology in articles published. Contact Kea Giles for additional information or assistance.

Given the many destructive forces at work after the death of an organism, it's remarkable that a fossil record even exists. This paradox has vexed paleontologists for 200 years: short-term experiments show disintegration within the first days to decades even for hard skeletons, and yet modern seabeds can contain abundant clam, snail, and coral shells, including some that are thousands of years old. New mathematical models applied to an extensive program of age-dating bivalve shells off the California coast go a long way to resolving this paradox. Authors Adam Tomasovych of the Slovak Academy of Sciences and colleagues from the U.S. show that not only do rates of shell loss decline over time, overturning the paradigm of a constant loss rate, but that this decline is abrupt rather than gradual. Occurring within the first ~500 years postmortem, this loss rises an extraordinary 100-fold in magnitude, with shells shifting from decadal to millennial half-lives, all within the upper few tens of centimeters of the seafloor. Fewer than 1% of all shells survive the initial postmortem phase, but the few that do are almost impervious to further destruction. This conclusion creates a paradigm shift, with great implications for the recycling and sequestration of biological carbonate, as well as for how we interpret the biological data encapsulated by fossil assemblages.

According to this study by Brian F. Atwater and colleagues, nearly forgotten research from decades ago casts doubt on some of the recent thinking about earthquake hazards in the Pacific Northwest. The hazards discussed are posed by the Cascadia subduction zone -- an active fault that slants eastward beneath the Pacific coast of southern British Columbia, Washington, Oregon, and northern California. Geologic studies in recent decades have provided increasingly specific estimates of Cascadia earthquake sizes and repeat times. The estimates affect public safety through seismic provisions in building design and tsunami limits on evacuation maps. The article does not question today's consensus that the Cascadia subduction zone produces enormous earthquakes repeatedly. Instead the article asks what geologists can say, with confidence, about details in the earthquake history. Which earthquakes represent long ruptures and which represent sequential, shorter breaks? Do earthquakes happen more often here than there? The article draws on Nixon-era findings about turbidites -- beds of sand and mud laid down by bottom-hugging, sediment-driven currents that infrequently emerged from submarine canyons onto the deep ocean floor. The article concludes that extracting Cascadia earthquake history from deep-sea turbidites is more complicated than was previously thought.

Large-volume eruptions of supervolcanoes are some of the most devastating natural disasters. We here demonstrates that at Yellowstone-type supervolcanoes, sub-volcanic silicic magma reservoirs are rapidly assembled and erupted on timescales of few thousand years; i.e., faster than resolvable by our best, currently available geochronology. We further present evidence that such supervolcanic magma reservoirs initially consist of a series of separated magma sub-chambers ("magma batches") that are generated by shallow crustal melting of previously erupted and buried, hydrothermally-altered tuffs and their plutonic equivalents. Co-existing magma batches may merge together and mix thoroughly forming voluminous bodies of eruptible magma. Merging of sub-chambers may even trigger such eruptions due to an increase in buoyancy overpressure. These results represent an alternative end-member model to supervolcanic magma reservoirs that consist of near solidus crystal mush, occupy the entire crustal volume under each caldera, and take several hundred thousand years to evolve to an eruptible state but suggest that magma chambers in the case of Yellowstone-type supervolcanoes are transient, short lived phenomena, also explaining why seismic methods have failed to find large volumes of crystal-poor eruptible magma underneath Yellowstone.

New geological observations documented that an event of global warming during the Cretaceous resulted in anoxic marine waters and deposition of hydrocarbon source rocks at ancient Arctic latitudes. In the Late Cretaceous, about 94 million years ago, Earth was characterized by high atmospheric carbon dioxide levels, a greenhouse climate with ice free poles and the widespread occurrence of shallow seas that covered wide areas of the continents. A globally observed event during this time, called "oceanic anoxic event 2," caused a dramatic expansion of oxygen deficient ocean waters and let to the widespread deposition of sediments at lower latitudes, which contain unusually large quantities of marine organic matter. In this study, it is demonstrated that the chain of feedbacks which lead to oceanic anoxic event 2 also resulted in anoxic waters and deposition of organic-rich sediments even at very high latitudes. These results are significant in two ways: First, they improve our knowledge of Arctic climate change during global warming. Second, they implicate widespread occurrence of hydrocarbon source rocks across the Arctic, which has previously been an uncertain parameter in hydrocarbon exploration.

Erosion rate and previous extent of interior layered deposits on Mars revealed by obstructed landslidesP.M. Grindrod, Dept. of Earth and Planetary Sciences, Birkbeck, University of London, Malet Street, London WC1E 7HX, UK, and Centre for Planetary Sciences at UCL/Birkbeck, London WC1E 6BT, UK; and N.H. Warner, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, 91109, USA, and Dept. of Geological Sciences, State University of New York at Geneseo, 1 College Circle, Geneseo, New York 14454, USA. Published online 29 July 2014; http://dx.doi.org/10.1130/G35790.1. OPEN ACCESS.

Layered deposits on Mars are important in studies of habitability because they not only preserve long sequences of Mars' history, but also exhibit evidence for past water. They are therefore high priority exploration targets, including the Mars Science Laboratory Curiosity rover, which currently sits near the base of an interior layered deposit in Gale Crater. Despite their importance, no consensus exists regarding how these layered deposits form, evolve, and whether they represent a complete record. This study identifies a unique location in Valles Marineris on Mars where layered deposits have obstructed landslides, before retreating by up to 2 km. The age of the landslides provides a time-stamp of formation that reveals a geologically-rapid erosion rate for these layered deposits, suggesting that they have been in a state of net loss for the last 200 to 400 million years. This high erosion rate is possible by wind abrasion if the deposits contain friable materials, or if sublimation of ice within the mound enhances modification. In either case, these examples in Valles Marineris indicate that the current form of layered deposits of similar morphology on Mars, such as Aeolis Mons in Gale Crate, under-represents their total depositional volume and likely the global sedimentary record.

Climatic control of the late Quaternary turbidite sedimentology of Lake Kivu, East Africa: Implications for deep mixing and geologic hazardsXuewei Zhang et al., Dept. of Earth Sciences, Syracuse University, Syracuse, New York 13203, USA. Published online 25 July 2014; http://dx.doi.org/10.1130/G35818.1.

Around the shores of Lake Kivu in the East African rift live two million people -- in Rwanda and the Democratic Republic of the Congo. Geologically ancient, Lake Kivu is the ancestral biological reservoir of thousands of endemic species observed in the other Great Lakes of East Africa. In close proximity to the lake is Mount Nyiragongo, one of the most active volcanoes of Africa, which is responsible for charging the deep waters of the lake with ~300 km3 of CO2 (at STP) and 60 km3 of CH4. A catastrophic release of these gases would have deadly consequences for the local population, and gas eruptions in the geologic past likely reduced the lake's biodiversity. By integrating subsurface geophysical data with radiocarbon-dated sediment cores, researchers show that deep density flows deposited "megaturbidites" numerous times over the past ~12 thousand years. The hyperpycnal flows originated during exceptional floods in wet climate intervals. The data suggest that extreme floods in Lake Kivu's recent history may have triggered deep mixing events, and that the extraordinary turbidity currents pose potential geologic hazards which are a risk to current efforts to extract the deep gas.

Most Earth sand deserts experience multidirectional winds that shape long, regularly spaced and parallel linear dunes with a well-defined orientation. In 1987, Rubin and Hunter experimentally showed that bedforms integrate contributions of successive winds and adopt an alignment that maximizes perpendicularity to sand transport. Depending on wind strength and direction, dunes are perpendicular (transverse), oblique or parallel (longitudinal) to the overall sand transport direction. The application of this model to aeolian dunes on Earth, using measured modern winds, reveals a clear mismatch for many linear dunes. This discrepancy is commonly ascribed to changes in wind regimes since many dunes date back to Last Glacial Maximum. Thus, dune alignment is used to constraint wind regimes at play in Earth past or on Mars and Titan where dunes are observed but wind measurements are missing. However, thanks to underwater experiments, authors demonstrate that a single wind regime can lead to two distinct dune orientations depending on sand availability. When growing from the destabilization of a sand bed, dunes indeed maximize perpendicularity to flows. In contrast, when sand supply is limited to localized sources, dunes extend in the mean flow direction. This observation completes the framework to understand dune orientation and their spreading through arid lands. Applied to Earth deserts, their model quantitatively predicts the orientation of both linear dunes and of their superimposed patterns, in agreement with the wind data of the last decades.

The largest extinction event of the past half billion years occurred during the Late Permian. Recent studies indicate that extreme global warming in the extinction's aftermath slowed ecosystem recovery during the Early Triassic. In addition, high temperatures potentially contributed to elevated continental erosion rates. This study aims to quantify changes in continental erosion rates through the use of strontium isotopes. Limestones deposited in the ancient Tethys Ocean during and after the extinction event were collected near Zal, Iran, and analyzed from their strontium isotopic composition. The results indicate that the highest erosion rates coincided with high global temperatures. As temperatures cooled, erosion rates declined, and under these more stable environmental conditions, ecosystems recovered.

Complex continental growth along the proto-Pacific margin of East GondwanaNicholas Rawlinson et al., School of Geosciences, Meston Building, University of Aberdeen, Aberdeen AB24 3UE, Scotland, UK. Published online 21 July 2014; http://dx.doi.org/10.1130/G35766.1.

Continental growth often takes place at convergent margins, where one tectonic plate descends (or subducts) beneath another. Mountain belts that form as a result of this process tend to be linear or mildly curved, but recent geodynamic modelling suggests that ingestion of a continental fragment at the subduction front can result in the formation of an orocline, or strongly curved mountain belt. In this study, Nicholas Rawlinson and colleagues find direct evidence of this process along the former proto-Pacific margin of East Gondwana in Australia by measuring seismic anisotropy (directional dependence of seismic wavespeed), which is sensitive to tectonic fabric that has been preserved in the crust. Images of seismic anisotropy reveal strong curvature in the upper to mid crust, consistent with a subducting slab wrapping around an embedded continental fragment. Their results suggest that crustal accretion is a complex process, and can involve entrainment of preexisting material, orocline formation and major strike-slip faulting, which are not normally considered in traditional models of subduction.

Some natural fault zones contain carbonaceous material (CM) even where host rocks do not contain it, and seismic fault motion can convert CM into graphite, which has marked low friction. Thus the origin of CM in fault zones is an important issue in fault mechanics. Previous high-velocity friction experiments have revealed various chemical reactions in fault zones during seismic fault motion, but most experiments have been conducted in an atmosphere under oxic conditions. In this study, Kiyokazu Oohashi and colleagues show for the first time that CM can form from calcite during high-velocity friction experiments under reducing conditions in an H2 atmosphere, and propose a new origin of CM in seismogenic fault zones. The gas analyses and temperature measurements showed that the abiotic CM formed as a result of high-temperature gas reactions during the experiments. The CM in the Nojima fault, southwestern Japan, resembles that produced in experiments, and hence the CM probably formed through the same processes. CM along a sliding surface in initially CM-free rocks may constitute geological evidence of seismic fault motion under reducing conditions. These findings of coseismic chemical reactions controlled by ambient redox conditions may open up new insights in both field observations and laboratory experiments.

Splay faults are large thrust faults emerging from the plate boundary to the seafloor in subduction zones. They are considered to enhance tsunami generation by transferring slip of the megathrust onto steeper faults, thus increasing vertical displacement of the seafloor. These structures are predominantly found offshore, and are therefore difficult to detect in seismicity studies, as most seismometer stations are located onshore. Lieser et al. combine ocean bottom seismometer and land-station observations from Central Chile in aftermath of the Mw 8.8 Maule earthquake in order to cover the northern part of the rupture area offshore as well as onshore. They are able to clearly image activity along a splay fault in the overriding plate of a subduction zone. Although the surface trace of the splay fault can be followed for a great part along the central Chilean margin only along a small part of it the splay fault is seismically active. The finding of splay faults being activated in some segments of the rupture zone but not others can be used to add additional constraints for slip distribution and tsunami modeling.

Zones of continent-continent collision involve deformation of the middle and lower crust. This involvement increases toward the central part of the collision zone, where temperature gets higher. Farther away from the collision zone the crust transitions into cooler and more rigid crust that is not internally affected by the collision-related deformation and subsequent extensional collapse. In the southern Scandinavian Caledonides, an ancient orogen that formed shortly before 400 million years ago, deep seismic data suggest that this transition is not completely gradual, but instead seems to be marked by two major shear zones, one in the upper and middle crust, and one in the lower crust/upper mantle. Across these extensional shear zones there is a marked thinning of the crust. We relate this thinning to crustal flow related to heating after the collision, and this crustal stretching predates rift formation in the North Sea.

From the abstract: The Paleocene-Eocene Thermal Maximum (PETM), about 56 million years ago, was a major global environmental perturbation attributed to a rapid rise in the concentration of greenhouse gases in the atmosphere. Geochemical records of tropical sea-surface temperatures (SSTs) from the PETM are rare and are typically affected by post-depositional diagenesis. To circumvent this issue, T. Aze and colleagues have analyzed oxygen isotope ratios of single specimens of exceptionally well-preserved planktonic foraminifera from the PETM in Tanzania.

From the abstract: The Barberton greenstone belt (BGB) includes eight known layers containing spherical particles (spherules) that condensed from rock vapor clouds formed by the impact of large meteorites or asteroids 3.25 to 3.47 billion years ago. Previous studies have inferred that the spherules represent bolides at least 20 to 70 km across. Spherule beds S1 to S4 have been previously characterized in detail: We provide here the first detailed analysis of more recently discovered beds S5 to S8.

From the abstract: The isotope composition of seawater sulfate is an important tracer of sulfur, carbon, and oxygen cycles in Earth's deep past. Carbonate-associated sulfate (CAS) extracted by acid digestion is widely used as a proxy for sulfate in paleo-seawater from which the carbonate minerals precipitated. Early and late diagenesis, weathering, and laboratory processing can in some cases compromise original seawater sulfate signals. Here, we report that extracted CAS can also be severely contaminated by recent atmospheric sulfate, especially when the sampled carbonates are from outcrops in arid to semi-arid climates or in heavily polluted regions.

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